The 1 = Dyna Gakki and 2 = Fujigen for the first number of the serial comes from a Japanese guy who was taking a guess.
It turns out that it's not true.
For instance, the Ibanez 70s guitars have Super 70s made by Maxon as were the Greco pickups, with a 1 and 2 first serial number.
Now I know from Jim Donahue who worked for Ibanez that Ibanez were never made by Dyna Gakki except for the Ibanez Blazer series.
So the 1 = Dyna Gakki and 2 = Fujigen = BS.
Also there are Grecos around with one pickup starting with a 1 and the other pickup starting with a 2 in the same guitar.
The 1 and 2 beginning pickup serial numbers are in both the YXXXX and MYYXXXX Greco guitar serial number formats.
The YXXXX and MYYXXXX Greco guitar serial number formats are both Fujigen's from looking at the guitars body routing.
The different YXXXX and MYYXXXX Greco guitar serial number formats might be due to different distribution around Japan.
Sometimes, Fujigen would install what pickups were hanging aound the factory.
I've seen a late 1970s Greco that had Super 70 pickups installed rather than the U-3000 or U-4000 pickups that the Greco was supposed to have had.
Super 70s, U-1000, Dry Z etc are all Maxon pickups up until late 1981.
The second number of the serial number is the year and the 3rd or 3rd and 4th number is the month (depending on if it's a 5 or 6 digit serial number, 5 digit serial = one digit for the month) and the end two numbers are the day.
Fujigen took over from Maxon as the pickup maker for Grecos and Ibanez's in late 1981 so most Greco Super Reals and 70 Grecos have Maxon made pickups and so do the 70s Ibanez guitars.
The Dry Z's might have a A3 magnet, which might be part of their sound.
The U-2000 seems to have a A5 magnet and the U-1000 seems to have a A8 magnet like the Super 70s.
Tracking down a A3 magnet and putting it in a U-2000 pickup might make it close to a Dry Z.
I don't really know because I havn't tried it.
I know that their are magnet difference between the low to high end Maxon Greco pickups but I'm not sure of any winding or wire differences.
If there is no difference between a Dry Z and a U-2000 pickup in terms of winding or wire differences then just changing magnets will make one become the other.
The original price difference of a Dry Z and a U-2000 was not huge so they might be very similar and maybe the same but with different magnets.
Dry 20,000 Yen
PU-2 15,000 Yen
U-2000 12,000 Yen
U-1000 10,000 Yen
Maxon seemed to have been copying the Gibson T-top pickups rather than Gibson PAF's for the Dry Z and U-1000 etc and Maxon used different magnets for the different pickup models.
The Maxon bobbin design looks like a Gibson T-top design and it makes a bit of sense that in the 70s Maxon would copy a Gibson T-top rather than a Gibson PAF as all the vintage BS wasn't really there in the 70s and the T-tops were more recent and the ones to copy.